RDL import via API

I am trying to work out what the basic Powershell API call is to import an RDL file to a workspace (.pbix works fine). I have seen several posts on this, none of them using Invoke-PowerBIRestMethod and none of them working. The error reporting via this API or via standard Invoke-RestMethod just returns blank 400 errors with no more information and nothing in the error stream and so I'm reduced to endless trial and error with no real information to go on. Here is an example (I have tried countless variants of this and this is just an example that returns a 400 error). I have tried every content-type, Url format etc. I can think of with no effect - just a blank 400 error. I am signed in and can run many other API calls without problem, including uploading .pbix files. Does anyone have an example of this working?

  $encoding = [System.Text.Encoding]::GetEncoding("iso-8859-1")
  $fileBytes = [System.IO.File]::ReadAllBytes($rdlfile)
  $boundary = [System.Guid]::NewGuid().ToString()
  $filename = Split-Path $rdlfile -Leaf
  $bodyt = @'
--{0}
Content-Disposition: form-data; name="fileData"; filename="{1}"
Content-Type: application/rdl

{2}
--{0}--

'@
  $body = $bodyt -f $boundary, $fileName, $encoding.GetString($fileBytes)

  try {
    $response = Invoke-PowerBIRestMethod -Url "groups/$wkspid/imports?nameConflict=$conflict" -Method Post -ContentType "multipart/form-data" -Body $body -Organization $tenantid
  }
  catch {
    Resolve-PowerBIError -Last
  }